This invention relates to the field of automobile alternator systems and, more particularly, to a protection circuit as for the tachometer of a diesel-powered vehicle.
In some systems, an output is taken from one or more phases of the alternator winding and supplied to various devices which operate from an AC output signal. The tachometer is one of the more important of these, and requires an AC signal with a significant amplitude in order to provide an accurate readout of engine speed. There are certain unusual circumstances, however, which can reduce the alternator output to a level which is unusable by the tachometer circuit. For example, if the battery of the automobile is fully or nearly charged, and the load goes from very light to very heavy, e.g., by turning on the headlights, the excitation will be increased in response to the current drain. If, at that point, the load is removed, there will be a considerable amount of energy stored in the alternator rotor. If the vehicle is a spark ignition engine, the ignition circuit will drain off this excess energy without any problem. However, if the engine is a diesel (with compression ignition), there is no equivalent means for dissipating this energy. The system, therefore, must attempt to store the energy in the battery, while the regulator will immediately reduce the field excitation to a minimum. This causes no problem within most automotive circuitry since the battery will be supplying all needed DC power. However, any circuit using the phase or AC output (such as the tachometer circuit) will be in trouble due to the sharply reduced alternator output. Since this problem is both infrequent and short-lived, it would be preferable to prevent the loss of excitation current under these circumstances, even at the expense of slight battery overcharge.
It would be advantageous if this prevention capability could be added to an existing circuit which examines the system and detects other "fault" conditions in the ignition system.
In a co-pending application, U.S. Ser. No. 06/687,603, assigned to the assignee of the present invention, a multifunction fault detector for use with the ignition system of a vehicle was disclosed. The present invention is preferably used in conjunction with that fault detector circuit.
In certain automobile markets, it is desired to provide for the detection and indication of a number of possible alternator system faults by means of a single lamp. Such a system must have, not only reliability, but essentially error-free operation. That is, temporary conditions of overvoltage or undervoltage which are not due to a fault requiring service are to be ignored, and apparent lack of shaft rotation or the transient pulses which are normal to automobile ignition systems must not cause false indications.
There are three main fault conditions which the system is able to detect and indicate; "non-rotation of the alternator", "overvoltage" and "undervoltage", and for any true fault, a lamp will be lit with a single accompanying legend such as "CHECK ENGINE". It is apparent that the fault lamp should not be lit for the temporary load change as described above.